The present invention is directed to integrated circuits. More particularly, the invention provides a method and apparatus for controlling a DC-DC converter used to supply power to various electronics and telephony devices. Merely by way of example, the invention provides techniques for generating a pulse width modulation (PWM) signal in a DC-DC converter and adjusting its duty cycle using multiple thresholds according to at least two separate schedules, namely a coarse tuning cycle and a fine tuning cycle. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be used to improve convergence efficiency of PWM signals in other applications.
DC to DC converters are often found in electronic devices. These devices include cellular phones and laptop computers which may require certain voltage levels different than supplied by the power source, or even negative voltages. In telephony applications, a subscriber line interface circuit (SLIC) often needs a high voltage to drive a POTS (plain old telephone service) phone.
DC to DC converters can be used to convert a DC voltage level to another. These circuits generally perform the conversion by first applying a DC voltage across an inductor or transformer for a period of time, which causes current to flow through it and store energy magnetically. During the time the DC voltage is switched off, the stored energy is transferred to an output terminal in a controlled manner. By adjusting the ratio of on/off time, also known as duty cycle, of the control pulse, the output voltage can be regulated even as the current demand changes. In addition, DC-DC converters can convert a DC input power to a DC output power while maintaining isolation between input and output, generally allowing differences in the input-output ground potentials. Further details of conventional DC-DC converters are provided throughout the present specification and more particularly below.
As an example, FIG. 1 shows a simplified schematic for a conventional DC-DC converter 100 capable of generating an output voltage either greater than or less than the input voltage. This DC converter is often useful in electronic or telephony applications The output voltage is adjustable based on the duty cycle of a switching transistor 120. A simplified explanation of a general operation of the converter is described below. While transistor 120 is in the ON state, input voltage source Vcc 115 is directly connected to inductor 130, accumulating energy in inductor 130. In this stage, capacitor 150 supplies energy to an output load connected to Vout 145. When transistor 120 is in the OFF state, inductor 130 is connected to capacitor 150 and an output terminal, so energy is transferred from inductor 130 to capacitor 150 and output terminal 145. The magnitude of output voltage is determined by the ratio of on/off times, or the duty cycle, of the pulse width modulated control signal 125.
In applications related to DC-DC converters, a feedback control loop can be included such that converter output tracks a predetermined reference voltage. In such an application, an error signal is first generated representing the offset between output voltage and target voltage. Then the error signal is compared with a triangular or saw tooth wave signal to generate a pulse-width modulated control signal, which can be used to control a switching device, such as transistor 120 in FIG. 1. Transistor 120 responds to the PWM signal to regulate output voltage to a predetermined voltage level.
Conventional pulse width modulators generally fall into one of two classes, analog PWM and digital PWM. In an analog PWM, control pulses are generated in analog circuits such as operational amplifiers and comparators, whereas in a digital PWM, control pulses are generated by digital logic circuits designed to implement a certain control algorithm. Analog PWMs generally provide more accurate pulse width control and faster response time. Unfortunately, conventional analog PWM controlled DC-DC converters suffer from certain limitations. For instance, conventional analog PWM controlled DC-DC converters can be susceptible to temperature and process variation and often consume more power and larger silicon area. There may also be other limitations with these conventional analog PWM controlled DC-DC converter.
To overcome certain limitations of the analog PWM converter, a digital PWM has been introduced. The Digital PWM can be used to avoid the complexity and high power consumption of its analog counterpart. In a digital PWM, a control signal is generated with a duty ratio in accordance with a difference between an output voltage and a target input voltage. A digital waveform generator is then used to produce a PWM signal having a duty cycle determined by the control signal. Alternatively, a digitized triangular waveform can be compared with a digitized output voltage to generate a digital PWM signal. Although successful, conventional digital PWM converters suffer from certain drawbacks. Conventional DC-DC converters utilizing digital PWMs tend to suffer from limited accuracy, because adjustment time step is limited by clock rate and incremental voltage change is dictated by digitization limitations. Further, conventional digital PWM converters tend to have relatively long response time, causing noise and output voltage overshoot or undershoot. These and other limitations of these conventional converters may be further explained throughout the present specification.
From the above, it is seen that an improved technique for controlling a DC-DC converter is desired.